1. Field of the Invention
The invention relates to an insulated wire (or enameled wire).
2. Description of the Related Art
Inverters are used as an efficient variable speed voltage controller in various electrical equipments. This type of inverter is controlled by a high-speed switching device in a frequency range of several KHz to several hundred KHz, and high surge voltage is generated when voltage is applied. The inverters in recent years can rise voltage rapidly especially by a high-speed switching device such as IGBT (Insulated Gate Bipolar Transistor), and instantaneous voltage up to twice the output voltage is generated as the surge voltage.
An enameled wire in which an insulating film (enamel film) is provided on a conductor is generally used as a material of a coil for electrical equipments using such an inverter. However, partial discharge may occur between the surfaces of the coiled enameled wire due to surge voltage, by which the enamel film is eroded. The erosion of the enamel film due to partial discharge eventually causes breakdown.
As a countermeasure against the influence of the surge voltage, it is desirable to use, e.g., a partial-discharge-resistant insulated wire (inverter-surge-resistant enameled wire) as is disclosed in, e.g., JP-A-2000-331539 and JP-A-2004-204187.
A conventional inverter-surge-resistant enameled wire has an enamel film formed around a conductor by applying and baking an organic/inorganic nanocomposite material thereon in which inorganic powder particles (silica, titania, alumina or zirconia, etc.) having a particle size of not more than 0.1 μm are dispersed in a polyamide-imide coating material or a polyester-imide coating material, etc., to be a base resin for enameled wire. An overcoat film for imparting mechanical strength, etc., is formed around the enamel film.
In such an enamel film, erosion thereof caused by partial discharge can be prevented. Note that, a nanocomposite is a composite material in which the dispersed inorganic power particles of not more than 0.1 μm are mixed to another material.